The trend took root in the 1980s when ultrasound technologies made it easier for families to detect fetal sex early and to abort if it was not what the parents desired, said the analysis in the Canadian Medical Association Journal.

Sons have traditionally been preferred over daughters in many parts of China, India and South Korea due to social, cultural and financial motivations. Sex-selective abortion is outlawed but can be difficult to enforce.The phenomenon was first spotted in South Korea in the early 1990s, when the sex ratio at birth (SRB) -- typically 105 male births to every 100 female births -- rose to 125 in some cities.

Similar rises in male births were seen in China, "complicated by the one-child policy, which has undoubtedly contributed to the steady increase in the reported SRB from 106 in 1979, to 111 in 1990, 117 in 2001 and 121 in 2005," said the study.

India has seen "sex ratios as high as 125 in Punjab, Delhi and Gujarat in the north but normal sex ratios of 105 in the southern and eastern states of Kerala and Andhra Pradesh," it added.